Use first squashed layer, then first from

If you were using a squashed base layer, the default insertion
point wasn't what it would be expected to be.

export.go

@@ -237,22 +237,30 @@ func (e *Export) ExtractLayers() error {
 	return nil
 }
 
-func (e *Export) FirstFrom() *ExportedImage {
+func (e *Export) firstLayer(pattern string) *ExportedImage {
 	root := e.Root()
 	for {
 		if root == nil {
 			break
 		}
 
 		cmd := strings.Join(root.LayerConfig.ContainerConfig.Cmd, " ")
-		if strings.Contains(cmd, "#(nop) ADD file") {
+		if strings.Contains(cmd, pattern) {
 			break
 		}
 		root = e.ChildOf(root.LayerConfig.Id)
 	}
 	return root
 }
 
+func (e *Export) FirstFrom() *ExportedImage {
+	return e.firstLayer("#(nop) ADD file")
+}
+
+func (e *Export) FirstSquash() *ExportedImage {
+	return e.firstLayer("#(squash)")
+}
+
 // Root returns the top layer in the export
 func (e *Export) Root() *ExportedImage {
 	return e.ChildOf("")

main.go

@@ -95,7 +95,11 @@ func main() {
 
 	}
 
-	start := export.FirstFrom()
+	start := export.FirstSquash()
+	// Can't find a previously squashed layer, use first FROM
+	if start == nil {
+		start = export.FirstFrom()
+	}
 	// Can't find a FROM, default to root
 	if start == nil {
 		start = export.Root()